This is the second phase of a project designed to combine PET imaging with pharmacologic challenge of serotonergic neurotransmission in order to study the effects of antidepressant treatment on serotonergic function in the brain. In this phase depressed patients will have a PET study of brain glucose utilization after administration of a serotonin-stimulating agent. This will be compared to placebo stimulation. The procedures will be performed at baseline and then again after antidepressant treatment.